As part of cell search process device need to find PSS, SSS and PBCH for MIB. In 5G all these combinedly termed as SSB(Synchronisation signal block). when device powers ON it primary job is to find SSB location through which will find PCI and tries to read MIB and SIB1 information.
PSS – Primary synchronisation signal which provides about frame boundary
SSS – Secondary synchronisation signal provides subframe boundary
PBCH – Physical broadcast channel gives MIB(master information block) information
So out of 20RBs or 240 sub carriers, PSS and SSS occupies 48 subcarriers each and rest are taken care by PBCH. but in order decode PBCH require a reference signal like in 4G we have cell specific ref signal which is not the case in 5G instead device uses PBCH DMRS(Demodulation reference signal) to PBCH and through which it reads Master information block.
PBCH DMRS occupies 25% of resources provided for PBCH.
to see in numbers for example:
20*2+8 = 48RBs in total for PBCH i.e 48*12 = 576 subcarriers
out of which DMRS takes 25% i.e 144 SCs and rest 432 for PBCH. PBCH uses QPSK modulation which is 2bits per symbol. in terms of Resource elements it will be 432*2 =864 resource elements.
Source: 3GPP 38.300
